Manual Overdrive Transmission Install - Mileage Maker

Swapping late-model automatic overdrive transmissions into vintage musclecars has become the hot ticket in recent years among hot rodders who want to enjoy the best of two worlds: deep gears for off-the-line acceleration and decent gas mileage for long-distance cruises.

With the proliferation of factory-offered automatic overdrive transmissions, these swaps have become fairly commonplace among those running juiceboxes. However, if you prefer banging gears manually, the alter-natives—such as a Richmond or ZF six-speed or a Gear Vendors overdrive unit—are usually frightfully expensive and could require a lot of costly fabrication.

But what if we told you that it’s possible to swap a four-speed manual overdrive transmission into your vintage Mopar for a fraction of the cost of an aftermarket transmission—simply by using junkyard and over-the-counter parts? If this claim leaves you scratching your head, you probably don’t know the story of Chrysler’s attempt to build a small, lightweight economy car at the height of the gas crisis.

In 1975, Chrysler debuted a manual overdrive version of the venerable A-833 four-speed transmission, which the company installed behind the equally venerable 225ci Slant Six in ’75 Plymouth Feather Dusters and Dodge Dart Lites.

The overdrive A-833 was basically a gearset change to the existing A-833 transmission, which had been in production since 1964. By flipping the 3-4 shift lever 180 degrees, engineers were able to keep the direct-drive Fourth gear and substitute a set of 0.73:1 Overdrive gears in place of the normal Third gear, without changing the shifter pattern. The introduction of these two efficient Mopars was a valiant attempt to compete with the droves of high-mileage Japanese subcompacts that were beginning to arrive on our shores around that time. Unfortunately, it was probably a case of too little, too late. Sadly, despite the fact that the manual overdrive feature enabled these lightweight cars to achieve 36 mpg on the highway, sales never really took off.

Today, Feather Dusters and Dart Lites are considered fairly scarce oddities. However, the manual overdrive A-833 survived in production long after the A-Body was dropped from Chrysler’s lineup at the end of the ’76 model year. The short-extension version of the transmission was used in F-Body Aspens and Volares until 1980, and the long-extension version was actually available in pickup trucks through 1989.

By virtue of their association with cars that—to put it gently—aren’t too high on many Top 10 Mopar lists, these transmissions still can be readily found in junkyards. In fact, even though four-speed cars of any kind are fairly scarce in boneyards here in Southern Cali-fornia, we found one in a ’76 Dodge Aspen station wagon in a local pull-your-own junkyard after only a few weeks of searching. At first, we were peeved that someone had beat us to the car and scored the shifter, linkage rods, and shift levers, but when the counter jockey at the yard mistook it for a Turbo 400 and only charged us $35 for the trans and bellhousing, we were all smiles and bragged at the Petersen offices in Los Angeles for days.

The intended recipient of this score was a ’67 Barracuda notchback coupe with a Slant Six and three-on-the-tree. Since it’s used as a daily driver, the prospect of higher mileage combined with moving the shifter from the column to the floor was a win-win proposition. Another benefit is the manual overdrive’s deep 3.09:1 First gear, which provides a hard launch.

Some research in the Mopar Performance Chassis book and other sources revealed that the swap would be a straightforward bolt-in. Surprisingly, all we had to do in the end to mount the transmission in the Barracuda was swap bellhousings, change the output shaft yoke, and buy a new U-joint.

What makes this swap particularly easy is the fact that many critical external dimensions of Mopar manual transmissions from the ’60s and ’70s are identical (see Manual Transmission Dimensions sidebar below). For example, the distance from the front face of the transmission case to the transmission mounting pad on a short-extension three- or four-speed gearbox is the same, so no crossmember modifications were necessary. Also, overall manual transmission lengths are the same, so no driveshaft modification is required if you keep the stock rearend.

That brings us to the one key difference between three- and four-speed transmissions: the output shaft yoke. Over the years, Chrysler used two different width yokes on its transmissions, driveshafts, and rearends—one for the large U-joint and one for the small U-joint. There are also two different transmission output shaft diameters—a 1.08-inch-diameter pinion for light-duty transmissions, such as the various three-speed manual boxes and the A-904 TorqueFlite, and a 1.25-inch-diameter pinion for the heavy-duty A-833 and A-727 TorqueFlite transmissions. Since we were swapping from a light-duty three-speed with the smaller diameter shaft, and used the small U-joint, we needed to obtain a new output shaft yoke with the larger-diameter pinion and the small U-joint to mate to our existing driveshaft. This part is readily available in the aftermarket (we ordered ours from Year One, along with some other parts necessary for the swap).

One odd thing about the manual overdrive A-833 is that it uses an enormous, 5.125-inch-diameter drive pinion bearing retainer, which fits into a corresponding pilot hole in the bellhousing. There are two ways to address this larger bearing retainer, if you plan to mate the transmission to a big- or small-block V8 bellhousing, which will have either a 4.354-inch or a 4.807-inch pilot hole, depending on the size of the drive pinion bearing that was in the original transmission. You can either swap bearing retainers on the trans, or have the bearing pilot hole in the bellhousing machined larger to fit the bigger retainer. New bearing retainers with the proper bolt pattern for the manual overdrive A-833 are still available through Mopar Performance. But if you’re installing the transmission behind a Slant Six—as we were—all you have to do is swap bellhousings to obtain the four-speed mounting bolt pattern, and bolt the transmission to it.

With all the parts assembled, the hardest part of the swap was finding a spare weekend to do it. Once it was completed, the results were immediate and dramatic. Highway fuel economy surged, the car became way more fun to drive with the four-speed Hurst shifter, and even with its anemic one-barrel carb, the Barracuda’s Leaning Tower of Power still produces enough torque to cruise up highway grades at 75-plus mph in Overdrive. The best part of this swap was that it was truly a direct bolt-in using stock, off-the-shelf parts, proof that Mother Mopar really knows how to plan ahead.

Column Shift to Floor Shift
Since mechanically, the swap used all factory parts, we wanted to preserve the factory-correct appearance of this swap by converting our column-shifted, three-speed car to a four-on-the-floor, without too much hacking. We were aided in this endeavor by several restoration companies, including Sherman & Associates, Year One, and Auto Custom Carpets, all of which furnished parts that allowed us to recreate the look of a factory four-speed.